1. Field of the Invention
This invention relates to a fuse resistor in which the heat of resistant film on an insulated substrate by overcurrent causes the substrate to break, so as to open a circuit, and to a discharging-type surge absorbing device with security mechanism comprising the fuse resistor and a discharging-type surge absorbing element.
2. Description of the Prior Art
Referring to FIG. 43, in a fuse resistor 150 a ruthenium paste and the like are spread as a heat generating resistant film 152 on the surface of an insulating substrate 151 made from ceramics such as alumina, forsterite, and like. An electrode pattern 153 formed at the both sides of resistant film 152 is connected to terminals 154. There is a triangle-shaped cutting 155 at the lower side of insulating substrate 151. (The term "cutting", as used in this application, is a literal translation of the Japanese original from which priority is claimed. A better term is "cutout" or "notch".)
Referring also to FIG. 44, terminals 154 are in a contact with an electronic circuit outside. For example, fuse resistor 150 is connected in series to discharging-type surge absorbing element 157 made from such as a gas arrester and the like between two lines A, A' constructed as a communication or power supply line to electronic circuit 156. When a big surge such as an inductive surge occurs at the lines, surge absorbing element 157 operates to absorb the surge. Also, when the lines are subjected to overvoltage that is more than a rated voltage of surge absorbing element 157 from an unexpected connection, the overcurrent causes heat generating resistant film 152 to heat, so that insulating substrate 15 1 is broken down along a line .alpha. (dash-line) from a top 158 of the cutting 155. Thus heat generating resistant film 152 is cut to open the circuit. The opening prevents surge absorbing element 157 from melting or igniting.
In another prior art example shown in FIG. 45, a discharging-type surge absorbing element with security mechanism 160 comprises an insulating substrate 161, a pair of heat generating resistant films 162 spread on a surface 161a of insulating substrate 161, a tiny discharging space 163 between resistant films 162, a pair of discharging electrodes 164 connected to resistant films 162 on a surface 16 1a of insulating substrate, a discharging space 165 between discharging electrode films 164, and a cover member 166 to cover surface 161a of insulating substrate 161, forming a space full of a discharged gas with surface 161 a.
This surge absorbing element 160 is connected to an electronic circuit through terminal film 167 connected with discharging electrode film 164. When a surge with more than a rated voltage of element 160 gets into the circuit, the potential difference between resistant films 162 generates a surface discharge at tiny space 163, thus producing a glow discharge by a priming effect. The increase of the surge current causes the glow discharge to be transformed into an arc discharge, so that the surge may be absorbed. Also, when the overvoltage continues to be applied to the circuit by an unexpected connection or the like, the overcurrent causes heat generating resistant film 162 to heat, and the substrate is broken down. As a result, air gets into the space full of the discharging gas. The arc discharge is disappeared at discharging space 165, opening the circuit.
In general, it is necessary to change a minimum shut-off current when breaking down insulating substrate 151 on fuse resistor 150, depending on the condition of the circuit. Let minimum shut-off current be I and resistance R, then heat amount W of resistant film 152 is expressed as follows: EQU W=I.sup.2 *R
Then, when heat amount W is fixed, minimum shut-off current I depends on resistance R. A smaller current should be used with a larger resistance. However, if the resistance becomes too big when use resistor 150 is connected in series on either of two lines A, A', the transmission loss increases. Moreover, if connected between lines A, A', an outer electronics device receives a high voltage. There is a drawback that it is hard to adjust the minimum shut-off current directly by the resistance of resistant films 152.
To protect a circuit from overcurrent, it is necessary to break down insulating substrate 151 completely in a short time when the surge happens. There is another drawback in the prior art that the substrate may not be broken down completely, permitting an overcurrent destroy the electronics device or the surge absorbing element 157 to ignite itself.
Also, in discharging-type surge absorbing element 160, the same problem occurs. Insulating substrate 161 is not broken down enough. A necessary circuit is damaged by continuous overcurrent.
As a further drawback, since fuse resistor 150 can not change the minimum shut-off current, there are needed plenty of fuse resistors if each minimum shut-off current has to be changed at different positions.